Stents With Radiopaque Markers

ABSTRACT

Various embodiments of stents with radiopaque markers arranged in patterns are described herein.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 11/796,226, filed on Apr. 26, 2007 which in turned claims benefit of provisional application No. 60/809,088, filed on May 26, 2006, both of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to implantable medical devices, such as stents. In particular, the invention relates to polymeric stents with radiopaque markers.

2. Description of the State of the Art

This invention relates to radially expandable endoprostheses, which are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices, which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.

The structure of stents is typically composed of scaffolding that includes a pattern or network of interconnecting structural elements or struts. The scaffolding can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape. In addition, a medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier. The polymeric scaffolding may also serve as a carrier of an active agent or drug.

The first step in treatment of a diseased site with a stent is locating a region that may require treatment such as a suspected lesion in a vessel, typically by obtaining an x-ray image of the vessel. To obtain an image, a contrast agent, which contains a radiopaque substance such as iodine is injected into a vessel. “Radiopaque” refers to the ability of a substance to absorb x-rays. The x-ray image depicts the lumen of the vessel from which a physician can identify a potential treatment region. The treatment then involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a bodily lumen to a region in a vessel that requires treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen. In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn allowing the stent to self-expand.

The stent must be able to simultaneously satisfy a number of mechanical requirements. First, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel lumen. In addition to having adequate radial strength or more accurately, hoop strength, the stent should be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material from which the stent is constructed must allow the stent to undergo expansion, which typically requires substantial deformation of localized portions of the stent's structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear thereon, including the cyclic loading induced by the beating heart. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.

In addition to meeting the mechanical requirements described above, it is desirable for a stent to be radiopaque, or fluoroscopically visible under x-rays. Accurate stent placement is facilitated by real time visualization of the delivery of a stent. A cardiologist or interventional radiologist can track the delivery catheter through the patient's vasculature and precisely place the stent at the site of a lesion. This is typically accomplished by fluoroscopy or similar x-ray visualization procedures. For a stent to be fluoroscopically visible it must be more absorptive of x-rays than the surrounding tissue. Radiopaque materials in a stent may allow for its direct visualization.

In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Therefore, stents fabricated from biodegradable, bioabsorbable, and/or bioerodable materials may be configured to meet this additional clinical requirement since they may be designed to completely erode after the clinical need for them has ended. Stents fabricated from biodegradable polymers are particularly promising, in part because they may be designed to completely erode within a desired time frame.

However, a significant shortcoming of biodegradable polymers (and polymers generally composed of carbon, hydrogen, oxygen, and nitrogen) is that they are radiolucent with no radiopacity. Biodegradable polymers tend to have x-ray absorption similar to body tissue.

One way of addressing this problem is to attach or couple radiopaque markers to a stent. The radiopaque markers allow the position of the stent to be monitored since the markers are can be imaged by X-ray imaging techniques. The ability to monitor or detect a stent visually is limited by the visibility of the markers.

SUMMARY OF THE INVENTION

Various embodiments of the present invention include a stent comprising radiopaque markers disposed on or within the stent, wherein the radiopaque markers are arranged longitudinally along an axis of the stent.

Further embodiments of the present invention include a stent comprising radiopaque markers disposed on or within the stent, wherein the radiopaque markers are arranged in a pattern along the circumference of the stent.

Additional embodiments of the present invention include a stent comprising a plurality of radiopaque markers disposed on or within on the stent, wherein the plurality of radiopaque markers are selectively arranged in a region of the stent to enhance the visibility of the stent with an imaging technique.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an exemplary stent.

FIG. 2 depicts an exemplary embodiment of a radiopaque marker and a section of a structural element of a stent with a depot for receiving the marker.

FIG. 3 depicts a stent pattern with radiopaque markers at proximal and distal ends.

FIG. 4 depicts a stent pattern with radiopaque markers arranged longitudinally along the cylindrical axis.

FIG. 5 depicts a stent pattern with radiopaque markers arranged in two longitudinally patterns along the cylindrical axis.

FIG. 6 depicts a stent pattern with radiopaque markers arranged in circumferential patterns.

FIG. 7 depicts a stent pattern with radiopaque markers arranged in a diagonal pattern.

FIG. 8 depicts a stent pattern with radiopaque markers arranged in circumferential patterns and longitudinal patterns.

FIG. 9 depicts a stent pattern with radiopaque markers selectively arranged in the proximal and distal regions of a stent.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be applied to stents and, more generally, implantable medical devices such as, but not limited to, self-expandable stents, balloon-expandable stents, stent-grafts, vascular grafts, cerebrospinal fluid shunts, pacemaker leads, closure devices for patent foramen ovale, and synthetic heart valves.

A stent can have virtually any structural pattern that is compatible with a bodily lumen in which it is implanted. Typically, a stent is composed of a pattern or network of circumferential and longitudinally extending interconnecting structural elements or struts. In general, the struts are arranged in patterns, which are designed to contact the lumen walls of a vessel and to maintain vascular patency. A myriad of strut patterns are known in the art for achieving particular design goals. A few of the more important design characteristics of stents are radial or hoop strength, expansion ratio or coverage area, and longitudinal flexibility. The present invention is applicable to virtually any stent design and is, therefore, not limited to any particular stent design or pattern. One embodiment of a stent pattern may include cylindrical rings composed of struts. The cylindrical rings may be connected by connecting struts.

In some embodiments, a stent of the present invention may be formed from a tube by laser cutting the pattern of struts in the tube. The stent may also be formed by laser cutting a polymeric sheet, rolling the pattern into the shape of the cylindrical stent, and providing a longitudinal weld to form the stent. Other methods of forming stents are well known and include chemically etching a polymeric sheet and rolling and then welding it to form the stent. A polymeric wire may also be coiled to form the stent. The stent may be formed by injection molding of a thermoplastic or reaction injection molding of a thermoset polymeric material. Filaments of the compounded polymer may be extruded or melt spun. These filaments can then be cut, formed into ring elements, welded closed, corrugated to form crowns, and then the crowns welded together by heat or solvent to form the stent. Lastly, hoops or rings may be cut from tubing stock, the tube elements stamped to form crowns, and the crowns connected by welding or laser fusion to form the stent.

FIG. 1 depicts an exemplary stent 100 with struts 110 that form cylindrical rings 115 which are connected by linking struts 120. The cross-section of the struts in stent 100 are rectangular-shaped. The cross-section of struts is not limited to what has been illustrated, and therefore, other cross-sectional shapes are applicable with embodiments of the present invention. The pattern should not be limited to what has been illustrated as other stent patterns are easily applicable with embodiments of the present invention.

A stent can be made of a biostable and/or biodegradable polymer. As indicated above, a stent made from a biodegradable polymer is intended to remain in the body for a duration of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. After the process of degradation, erosion, absorption, and/or resorption has been completed, no portion of the biodegradable stent, or a biodegradable portion of the stent will remain. In some embodiments, very negligible traces or residue may be left behind. The duration can be in a range from about a month to a few years. However, the duration is typically in a range from about one month to twelve months, or in some embodiments, six to twelve months. It is important for the stent to provide mechanical support to a vessel for at least a portion of the duration. Many biodegradable polymers have erosion rates that make them suitable for treatments that require the presence of a device in a vessel for the above-mentioned time-frames.

As indicated above, it is desirable to have the capability of obtaining images of polymeric stents with x-ray fluoroscopy during and after implantation. Various embodiments of the present invention include stents with markers arranged in patterns or selectively arranged on the stent in a manner that facilitates visualization of the stent.

Various types of markers can be used in embodiments of the present invention. Representative types of markers include constructs made of a radiopaque material that is disposed within depots or holes in a stent. The construct can be, but is not limited to a pellet, bead, or slug. The depot or hole can be made to accommodate the shape of the marker. In an embodiment, the depot may be formed in a structural element by laser machining. The depot may extend partially or completely through the portion of the stent. For example, an opening of a depot may be on an abluminal or luminal surface and extend partially through the stent or completely through to an opposing surface. The markers may be sufficiently radiopaque for imaging the stent. In addition, embodiments of the stents with markers should be biocompatible and should not interfere with treatment.

FIG. 2 illustrates an exemplary embodiment of a spherical marker 150 and a section 155 of a structural element of a stent with a cylindrical depot 160. Depot 160 accommodates the shape of spherical marker 150 so that is can be positioned within depot 160. Markers can be attached or coupled to a stent using various techniques, including, but not limited to, gluing, welding, or through an interference fit.

The markers and manner of positioning on the stent are merely representative. Embodiments of the present invention are not limited to the type of marker or the manner of attachment or coupling to the stent. The present invention applies to markers that can be attached or coupled in, on, or around a stent at a specific locations or positions on the stent structure or geometry.

In general, increasing the size of a marker enhances the visibility of a stent. However, increasing the size of a marker can have disadvantages. For example, a larger marker can result in an undesirably large profile of the stent which can interfere with the flow of blood in a vessel. Complications such as thrombosis can result from the disturbed blood flow. Additionally, a larger marker disposed in a structural element can negatively affect its structural integrity.

Embodiments of the present invention are directed to positioning or arranging markers on a stent to facilitate detection or monitoring the position of the stent. In certain embodiments, the markers can be arranged in a geometrical pattern that facilitates visualization of the stent.

FIG. 3 depicts a stent pattern 180 in a flattened condition showing an abluminal or luminal surface so that the pattern can be clearly viewed. When the flattened portion of stent pattern 180 is in a cylindrical condition, it forms a radially expandable stent. Line A-A corresponds to the longitudinal axis of a stent made from stent pattern 180 and line B-B corresponds to the circumferential direction of a stent made from stent pattern 180.

Stent pattern 180 includes cylindrically aligned rings 185 and linking structural elements 190. Structural elements at a proximal end 205 and distal end 210 of stent pattern 180 include depots with pairs of radiopaque markers 195 and 200, respectively, disposed within the depots. As shown FIG. 3, the structural elements are thicker in the vicinity of markers 195 and 200 to compensate for the presence of the depots. In general, it is desirable to place radiopaque markers in regions of a stent pattern that experience relatively low strain during crimping and expansion. Such low strain regions include straight portions of structural elements and “spider regions” which are intersections of three or more structural elements.

A physician can monitor the position of the stent due to the presence of the radiopaque markers which are visible using X-ray imaging. Since markers are located at the distal and proximal ends of the stent, the positions of the markers allow the physician to locate the ends of the stent. However, the small size of the markers can make it difficult to visually detect the individual markers. As indicated above, the size of markers is limited by a desired profile of the stent and structural integrity of structural elements. Since the markers are separated by the length of the stent, locating the ends of the stent can be difficult.

Various embodiments of the present invention include a stent having radiopaque markers arranged in patterns or selectively arranged in a region in a manner that enhances or facilitates visualization of the stent. Radiopaque markers arranged in patterns or selectively arranged in particular region(s) have greater visibility than one or two localized markers and can substantially enhance the visibility of a stent.

FIG. 4 depicts an exemplary stent pattern 220 of the present invention. Stent pattern 220 is the same as pattern 180 in FIG. 3 except for the number and arrangement of radiopaque markers. Stent pattern 220 includes radiopaque markers 225 which are arranged longitudinally along the cylindrical axis of stent pattern 220. Radiopaque markers 225 can be placed in or on any portion of the structural elements of stent pattern 220, as long as the mechanical integrity of the structural element is not undesirably compromised. As shown in FIG. 4, radiopaque markers 220 are located in the “spider regions,” which are relatively low strain regions of stent pattern 220. Such a pattern is substantially more visible than one or two markers localized at either end of the stent, as depicted in FIG. 3.

The longitudinal pattern of markers 225 extends from a proximal end 230 to a distal end 235 of stent pattern 220. In some embodiments, the pattern does not extend all the way between the proximal end and distal end. A portion between the proximal and distal ends can be devoid of markers.

In other embodiments, the visibility of the stent can be further enhanced by including additional marker patterns. For example, FIG. 5 depicts another exemplary stent pattern 250 of the present invention that is the same as the stent patterns of FIGS. 3 and 4 except for the number and arrangement of radiopaque markers. Stent pattern 250 includes a longitudinal pattern 255 of markers 265 and another longitudinal pattern 260 of markers 270 at a different circumferential position. Marker patterns 255 and 260 are separated by a circumferential distance or arc D. D can be between 0° and 45°, 45° and 90°, 90° and 135°, and between 135 and 180°.

Further embodiments can include marker patterns along at least a portion of the circumference. Such marker patterns can include, but are not limited to, a circular pattern, diagonal pattern, or a spiral pattern. FIG. 6 depicts another exemplary stent pattern 280 of the present invention that is the same as the previous stent patterns except for the number and arrangement of radiopaque markers. Stent pattern 280 includes three circumferential marker patterns: a marker pattern 285 with markers 287 at a proximal end 300, a marker pattern 290 with markers 292 at a distal end 305, and a marker pattern 295 with markers 297 between proximal end 300 and distal end 305. These marker patterns substantially enhance the visibility of the ends of the stent as compared to one or two markers at the ends, such as that illustrated in FIG. 3.

Each of the circumferential patterns extends all the way around the circumference of a stent made from stent pattern 280 and is positioned at a single axial position. Alternatively, the circumferential patterns can extend partially around the circumference. Circumferential patterns can also extend diagonally around the circumference so that the marker pattern is not a single axial position. For example, FIG. 7 depicts an exemplary stent pattern 320 that has a diagonal marker pattern with markers 325 extending between a proximal end 330 to a distal end 335.

In some embodiments, a stent can include both longitudinal and circumferential marker patters. FIG. 8 depicts an exemplary stent pattern 350 of the present invention that is the same as the previous stent patterns except for the number and arrangement of radiopaque markers. Stent pattern 350 includes two circumferential marker patterns, 355 and 360, and two longitudinal marker patterns, 365 and 370. Marker patterns 355 and 360 are at proximal end 375 and 380. Such a combination of patterns enhances the visibility of both the ends of the stent as well as the longitudinal extent of the stent.

In certain embodiments, a plurality of radiopaque markers can be selectively arranged in a region of the stent to enhance the visibility of the stent with an imaging technique. For example, the markers can be selectively arranged at a proximal region, distal region, or both. FIG. 9 depicts an exemplary stent pattern 400 of the present invention that is the same as the previous stent patterns except for the number and arrangement of radiopaque markers. Stent pattern 400 includes a group of markers 405 arranged at a proximal end 415 and a group of markers 410 arranged at a distal end 420. The groups of markers tend to increase the visibility of the respective ends of the stent as compared to the markers is FIG. 3. The individual markers can be selected and coupled in a way that there is little or no negative effect on the structural integrity of the stent and also with a relatively low profile.

As indicated above, a stent may have regions with a lower strain than other higher strain regions when the stent is placed under an applied stress during use. A depot for a radiopaque marker may be selectively positioned in a region of lower strain. The selected region of the structural element may be modified to have a higher mass or thickness than a region of lower strain without a marker so as to maintain the load-bearing capability of the region and to inhibit decoupling of the marker from the stent.

Furthermore, the markers may be coupled to any desired location on a stent. In some embodiments, it may be advantageous to limit the placement of a marker to particular locations or portions of surfaces of a stent. For example, it may be desirable to couple a marker at a sidewall face of a structural element to reduce or eliminate interference with a lumen wall or interference with blood flow, respectively. To delineate just the margins of the stent so that the physician may see its full length, markers can be placed only at the distal and proximal ends of the stent.

As indicated above, a stent may include a biostable and/or a biodegradable polymer. The biodegradable polymer may be a pure or substantially pure biodegradable polymer. Alternatively, the biodegradable polymer may be a mixture of at least two types of biodegradable polymers. The stent may be configured to completely erode away once its function is fulfilled.

In certain embodiments, the marker may be biodegradable. It may be desirable for the marker to degrade at the same or substantially the same rate as the stent. For instance, the marker may be configured to completely or almost completely erode at the same time or approximately the same time as the stent. In other embodiments, the marker may degrade at a faster rate than the stent. In this case, the marker may completely or almost completely erode before the body of the stent is completely eroded.

Furthermore, a radiopaque marker may be composed of a biodegradable and/or biostable metal. Biodegradable or bioerodable metals tend to erode or corrode relatively rapidly when exposed to bodily fluids. Biostable metals refer to metals that are not biodegradable or bioerodable or have negligible erosion or corrosion rates when exposed to bodily fluids. Additionally, it is desirable to use a biocompatible biodegradable metal for a marker. A biocompatible biodegradable metal forms erosion products that do not negatively impact bodily functions.

In one embodiment, a radiopaque marker may be composed of a pure or substantially pure biodegradable metal. Alternatively, the marker may be a mixture or alloy of at least two types of metals. Representative examples of biodegradable metals for use in a marker may include, but are not limited to, magnesium, zinc, tungsten, and iron. Representative mixtures or alloys may include magnesium/zinc, magnesium/iron, zinc/iron, and magnesium/zinc/iron. Radiopaque compounds such as iodine salts, bismuth salts, or barium salts may be compounded into the metallic biodegradable marker to further enhance the radiopacity. Representative examples of biostable metals can include, but are not limited to, platinum and gold.

In some embodiments, the composition of the marker may be modified or tuned to obtain a desired erosion rate and/or degree of radiopacity. For example, the erosion rate of the marker may be increased by increasing the fraction of a faster eroding component in an alloy. Similarly, the degree of radiopacity may be increased by increasing the fraction of a more radiopaque metal, such as iron, in an alloy. In one embodiment, a biodegradable marker may be completely eroded when exposed to bodily fluids, such as blood, between about a week and about three months, or more narrowly, between about one month and about two months.

In other embodiments, a radiopaque marker may be a mixture of a biodegradable polymer and a radiopaque material. A radiopaque material may be biodegradable and/or bioabsorbable. Representative radiopaque materials may include, but are not limited to, biodegradable metallic particles and particles of biodegradable metallic compounds such as biodegradable metallic oxides, biocompatible metallic salts, gadolinium salts, and iodinated contrast agents.

In some embodiments, the radiopacity of the marker may be increased by increasing the composition of the radiopaque material in the marker. In one embodiment, the radiopaque material may be between 10% and 80%; 20% and 70%; 30% and 60%; or 40% and 50% by volume of the marker.

The biodegradable polymer in the marker may be a pure or substantially pure biodegradable polymer. Alternatively, the biodegradable polymer may be a mixture of at least two types of biodegradable polymers. In one embodiment, the composition of the biodegradable polymer may be modified to alter the erosion rate of the marker since different biodegradable polymers have different erosion rates.

A biocompatible metallic salt refers to a salt that may be safely absorbed by a body. Representative biocompatible metallic salts that may used in a marker include, but are not limited to, ferrous sulfate, ferrous gluconate, ferrous carbonate, ferrous chloride, ferrous fumarate, ferrous iodide, ferrous lactate, ferrous succinate, barium sulfate, bismuth subcarbonate, bismuth potassium tartrate, bismuth sodium iodide, bismuth sodium tartrate, bismuth sodium triglycollamate, bismuth subsalicylate, zinc acetate, zinc carbonate, zinc citrate, zinc iodate, zinc iodide, zinc lactate, zinc phosphate, zinc salicylate, zinc stearate, zinc sulfate, and combinations thereof. The concentration of the metallic salt in the marker may be between 10% and 80%; 20% and 70%; 30% and 60%; or 40% and 50% by volume of the marker.

In addition, representative iodinated contrast agents may include, but are not limited to acetriozate, diatriozate, iodimide, ioglicate, iothalamate, ioxithalamate, selectan, uroselectan, diodone, metrizoate, metrizamide, iohexol, ioxaglate, iodixanol, lipidial, ethiodol, and combinations thereof. The concentration of an iodinated contrast agent in the marker may be between 5% and 80%; 20% and 70%; 30% and 60%; or 40% and 50% by volume of the marker.

The composition of metallic particles may include at least those biodegradable metals discussed above as well as metallic compounds such as oxides. The concentration of metallic particles in the marker may be between 10% and 80%; 20% and 70%; 30% and 60%; or 40% and 50% by volume of the marker. Additionally, individual metallic particles may be a pure or substantially pure metal or a metal compound. Alternatively, individual metallic particles may be a mixture of at least two types of metals or metallic compounds. Individual metallic particles may also be a mixture or an alloy composed of at least two types of metals.

In certain embodiments, the metallic particles may be metallic nanoparticles. A “nanoparticle” refers to a particle with a dimension in the range of about 1 nm to about 100 nm. A significant advantage of nanoparticles over larger particles is that nanoparticles may disperse more uniformly in a polymeric matrix, which results in more uniform properties such as radiopacity and erosion rate. Additionally, nanoparticles may be more easily absorbed by bodily fluids such as blood without negative impact to bodily functions. Representative examples of metallic particles may include, but are not limited to, iron, magnesium, zinc, platinum, gold, tungsten, and oxides of such metals.

In one embodiment, the composition of different types of metallic particles in the mixture as well as the composition of individual particles may be modified to alter erosion rates and/or radiopacity of the marker. In addition, the ratio of polymer to metallic particles may be modified to alter both the erosion rate, and radiopacity.

A marker may be fabricated by methods including, but not limited to, molding, machining, assembly, or a combination thereof. All or part of a metallic or polymeric marker may be fabricated in a mold or machined by a method such as laser machining.

In general, polymers can be biostable, bioabsorbable, biodegradable, or bioerodable. Biostable refers to polymers that are not biodegradable. The terms biodegradable, bioabsorbable, and bioerodable, as well as degraded, eroded, and absorbed, are used interchangeably and refer to polymers that are capable of being completely eroded or absorbed when exposed to bodily fluids such as blood and can be gradually resorbed, absorbed and/or eliminated by the body.

Biodegradation refers generally to changes in physical and chemical properties that occur in a polymer upon exposure to bodily fluids as in a vascular environment. The changes in properties may include a decrease in molecular weight, deterioration of mechanical properties, and decrease in mass due to erosion or absorption. Mechanical properties may correspond to strength and modulus of the polymer. Deterioration of the mechanical properties of the polymer decreases the ability of a stent, for example, to provide mechanical support in a vessel. The decrease in molecular weight may be caused by, for example, hydrolysis, oxidation, enzymolysis, and/or metabolic processes.

Representative examples of polymers that may be used to fabricate embodiments of stents, or more generally, implantable medical devices include, but are not limited to, poly(N-acetylglucosamine) (Chitin), Chitosan, poly(3-hydroxyvalerate), poly(lactide-co-glycolide), poly(3-hydroxybutyrate), poly(4-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), polyorthoester, polyanhydride, poly(glycolic acid), poly(glycolide), poly(L-lactic acid), poly(L-lactide), poly(D,L-lactic acid), poly(D,L-lactide), poly(L-lactide-co-D,L-lactide), poly(caprolactone), poly(L-lactide-co-caprolactone), poly(D,L-lactide-co-caprolactone), poly(glycolide-co-caprolactone), poly(trimethylene carbonate), polyester amide, poly(glycolic acid-co-trimethylene carbonate), co-poly(ether-esters) (e.g. PEO/PLA), polyphosphazenes, biomolecules (such as fibrin, fibrinogen, cellulose, starch, collagen, and hyaluronic acid), polyurethanes, silicones, polyesters, polyolefins, polyisobutylene and ethylene-alphaolefin copolymers, acrylic polymers and copolymers, vinyl halide polymers and copolymers (such as polyvinyl chloride), polyvinyl ethers (such as polyvinyl methyl ether), polyvinylidene halides (such as polyvinylidene chloride), polyacrylonitrile, polyvinyl ketones, polyvinyl aromatics (such as polystyrene), polyvinyl esters (such as polyvinyl acetate), acrylonitrile-styrene copolymers, ABS resins, polyamides (such as Nylon 66 and polycaprolactam), polycarbonates, polyoxymethylenes, polyimides, polyethers, polyurethanes, rayon, rayon-triacetate, cellulose acetate, cellulose butyrate, cellulose acetate butyrate, cellophane, cellulose nitrate, cellulose propionate, cellulose ethers, and carboxymethyl cellulose. Additional representative examples of polymers that may be especially well suited for use in fabricating embodiments of implantable medical devices disclosed herein include ethylene vinyl alcohol copolymer (commonly known by the generic name EVOH or by the trade name EVAL), poly(butyl methacrylate), poly(vinylidene fluoride-co-hexafluoropropene) (e.g., SOLEF 21508, available from Solvay Solexis PVDF, Thorofare, N.J.), polyvinylidene fluoride (otherwise known as KYNAR, available from ATOFINA Chemicals, Philadelphia, Pa.), ethylene-vinyl acetate copolymers, poly(vinyl acetate), styrene-isobutylene-styrene triblock copolymers, and polyethylene glycol.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications can be made without departing from this invention in its broader aspects. Therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention. 

1-20. (canceled)
 21. A method of crimping a stent on a balloon, comprising: (a) providing a biodegradable polymeric stent, wherein the material from which structural elements of the stent are made consist of poly(l-lactide) and a second, different polymer (“PLLA-based polymer”); (b) attaching markers to the polymeric stent to provide capability of obtaining images of the PLEA-based polymer with an imaging device during and after implantation of the stent in a vessel, wherein the stent consists of two pairs markers, two of the markers being attached in adjacent configuration about a proximal most end ring of the stent, and the remaining two markers being attached in an adjacent configuration about a distal most end ring of the stent, with the proviso that the stent has no other markers but for the four markers to allow for imaging of the PLLA-based polymer and with a further proviso that an entire length of the stent between the first pair and second pair of markers is devoid of any markers, wherein a center point of a first marker is positioned at a circumferential distance from a center point of its adjacent second marker, wherein a center point of a third marker is positioned at a circumferential distance from a center point of its adjacent fourth marker, and wherein the pair of markers about the proximal most end ring of the polymeric stent are circumferentially off-set from the pair of markers about the distal most end ring of the polymeric stent; and (c) crimping the stent on a balloon of a catheter assembly by application of a crimping pressure to the stent positioned over the balloon, wherein when the crimping pressure is applied to the stent, the stent has regions of low strain and regions of high strain, such that PLLA-based elements to which each pair of marker is attached are regions of low strain, and wherein the PLLA-based elements of the markers do not compromise the mechanical integrity of the stent. 